1. Field of the Invention
The present invention relates to an engine cooling water control system.
2. Description of the Related Arts
In conventional automotive engine cooling water control systems for example, the temperature of cooling water within the engine is controlled to a control objective temperature which has previously been set in conformity with the driving condition such as engine load or engine speed.
FIG. 4 illustrates a schematic configuration of an automotive engine cooling water control system. The automotive engine cooling water control system has a structure in which a cooling water control valve 21 opens in response to a control signal from an ECM 20, allowing cooling water which has been deprived of heat by a radiator 22 to be circulated by a water pump 23 through an inlet flow passage 24 into a water jacket 25 of the engine, the cooling water cooling a cylinder block 26 and a cylinder head 27 and thereafter being returned through an outlet flow passage 28 to the radiator 22 for radiation, the cooling water being again circulated into the water jacket 25. The cooling water flow passage within the water jacket 25 further includes a bypass flow passage 29 and a heater flow passage 31. The bypass flow passage 29 allows cooling water which has been warmed as a result of cooling the cylinder block 26 and the cylinder head 27 to again circulate into the water jacket 25 to thereby reduce variations in water temperature and water pressure. The heater flow passage 31 allows a circulation of cooling water between the water jacket 25 and a heater 30 for warming the automobile interior space in the cold. The heater flow passage 32 is provided with a heater cut valve 32 for cutting off the flow of cooling water if necessary.
In such a cooling water control system the temperature of cooling water within the engine is detected by a water temperature sensor so that the valve opening of the cooling water control valve 21 is regulated depending on the detected temperature to provide a control of the circulation flow rate of cooling water to the radiator 22, thereby controlling the temperature of the cooling water within the engine to a predetermined control objective temperature. At that time, the control objective temperature is set in compliance with the driving condition such as the engine load or engine speed to thereby improve the fuel efficiency, exhaust performance and drive performance. In other words, the known system attempts to improve the engine power and to secure the reliability in the high load engine driving condition whereas it attempts to achieve a reduction in friction and an improvement in combustion in the low load engine driving condition.
Its outline will hereinafter be described. Referring to FIG. 5 there is illustrated a relationship among the engine speed, torque and accelerator opening (Road, load line: RL) in the respective engine driving conditions which are largely separated into regions A and B. The region A denotes a high load condition in which the torque value is high relative to the engine speed with a large accelerator opening. The region B is a one indicative of a low load condition in which the torque value is lower than in the region A relative to the engine speed and in which the torque value decreases accordingly as the engine speed rises with a gently increased accelerator opening for a gentle acceleration. Then, in the ECM 20 the control objective temperature is set to a high temperature (e.g., 110 degrees centigrade) when the driving condition is in the region B whereas it is set to a low temperature (e.g., 85 degrees centigrade) when the driving condition is in the region A, in response to which the cooling water control value 21 is opened or closed to control the amount of circulation of the cooling water.
However, even though the driving condition of the engine has been separated into the high load condition (region A) and the low load condition (region B) as described above to set separately different control objective temperatures, it will be impossible to effect instantaneous replacement of cooling water within the water jacket 25 with cooling water within the radiator 22 when for example the control objective temperature of the cooling water within the engine has been switched from high temperature to low temperature. For this reason, the cooling water within the engine takes much time to lower its temperature, resulting in a poor responsibility upon the switching control of the cooling water temperature within the engine, which will put obstacles to improvement in the fuel efficiency, power, reliability, etc.
In case of the one having the configuration shown in FIG. 4 in particular, cooling water circulated through the bypass flow passage 29 will raise the temperature of cooling water flowing from the radiator 22 into the water jacket 25, which also makes the lowering of the cooling water temperature within the engine quite time consuming. Thus, it caused a further lowering of the responsibility upon the switching control of the cooling water temperature within the engine.